Objective assessment of lung function has provided valuable insights into the normal process of growth and development of infant lungs and airways. It is also an important component in the diagnosis and management of respiratory diseases and disorders, and is essential to our understanding of their acute or chronic effects and eventually our ability to prevent or minimize these effects (American Thoracic Society/European Respiratory Society. Respiratory mechanics in infants: physiological evaluation in health and disease. Am Rev Respir Dis 1993; 147: 474-96.).
The functional residual capacity (FRC), i.e., the volume of air in the lungs and airways at end-tidal expiration, is the only lung volume that is routinely measured in infants. (American Thoracic Society/European Respiratory Society. Respiratory mechanics in infants: physiological evaluation in health and disease. Am Rev Respir Dis 1993; 147: 474-96; Gaultier C. Lung volume in neonates and infants. Eur Respir J 1989; 2(Suppl 4: 130s-4s)). In infants, FRC is usually dynamically determined, in that young infants inspire before expiration ends passively. (LeSouef PN, England S J, Bryan A C. Passive respiratory mechanics in newborns and children. Am Rev Respir Dis 1984; 129, 552-56.). Therefore, FRC is an unreliable volume landmark known to shift with many dynamic events, including the airway caliber (Maxwell D L, Prendiville A, Rose A, et al. Lung Volume changes during histamine-induced bronchoconstriction in recurrently wheezy infants. Pediatr Pulmonol 1988; 5,145-151), sleep state (Stark A R, Cohlan B A, Waggener T B, et al. Regulation of end-expiratory lung volume during sleep in premature infants. J Appl Physiol 1987; 62; 1117-23; Beardsmore C S, MacFayden U M, Moosavi S S, et al. Measurement of lung volume during active and quiet asleep in infants. Pediatr Pulmonol 1989; 7:71-77.) and the addition of dead space (Stick S M, Arnott J, Turner D J, et al. Bronchial responsiveness and lung function in recurrently wheezy infants. Am Rev Respir Dis 1991; 144, 1012-15). Previous measurements of other lung volumes such as the residual volume (RV); i.e., the volume of air remaining in the lung at the end of a forced expiration, and total lung capacity (TLC) either required invasive techniques or had an unacceptable reproducibility.